Are Ediacaran Ambient Inclusion Trails Trace Fossils?

Ambient Inclusion Trails (AITs) are intriguing microtubular structures that commonly occur in association with pyrite in Precambrian organic-rich cherts and phosphorites. They are thought to be created by the migration of pyrite or other crystal grains through a lithified substrate driven by pressure solution from the in situ thermal decomposition of organic matter. New phosphorite samples of the Ediacaran Doushantuo Formation (South China) contain abundant AITs exhibiting diverse morphotypes, which may be distinguished from filamentous microfossils and endolithic microborings with a suite of morphological criteria based on optical microscopy and scanning electron microscopy (SEM). Black shales of the Baizhu section contains abundant pyrite framboids whose size distributions reveal significant temporal variations of redox conditions in shallow marine waters that probably promoted the formation of the Doushantuo phosphorites.

AITs in the phosphorites are categorized into three types and further into five subtypes (I-a, I-b, II-a, II-b, and III) based on their morphologies and observed or interpreted associations with various kinds of terminal pyrite crystals. Among these, subtype II-a, single striated microtubes 2–10 μm wide, are interpreted to have resulted from migration of intact pyrite framboids. Those of subtype II-b, dense clusters of outward radiating microtubes with consistent widths and inward-facing cuspate ridges, likely have formed by explosive disintegration and propulsion of pyrite framboids due to highly concentrated carbon dioxide gas during the oxidation of organic matter. During early diagenesis, formation of euhedral and framboidal pyrites involve a suite of biogeochemical and physical processes including non-biological oxidation of organic matter and reduction of sulfate in the presence of ferrous iron. Following the burial of pyrites, further oxidative degradation of organic matter produced abundant CO2 gas, which drives the pyrites to move through the solid, but not yet fully lithified phosphatic gel composing granules. This model explains the formation of previously reported but unexplained star-burst type AITs and it may be tested by experimental studies.

Our new observations provide evidence for the widespread occurrence of AITs in the Doushantuo phosphorites and urge careful petrographic examinations and differentiation between AITs and morphologically similar biogenic microstructures.